The present invention relates to an electric machine having a stator, a rotor, which interacts magnetically with the stator, a housing, which surrounds the stator and the rotor, and a hollow shaft on which the rotor is arranged and which is supported on the housing.
A stator of an electric machine, which heats due to losses, can be relatively easily cooled directly by means of an air or water cooling system. The rotor of an electric machine can only be cooled directly if the motor housing is open. This requirement of an open motor housing nevertheless in some instances contravenes a special protection system of the electric machine. In particular, electric machines with explosion protection require the housing to be closed. In this case, a direct cooling of the rotor can then not be realized with previously known techniques.
The thermal energy produced due to losses in the rotor can also be output indirectly to the motor housing by transmission using air. This indirect cooling principle is however not as efficient as a direct cooling system. Furthermore, the stator is heated due to the rotor losses. A consequence of this heating-up of the electric machine is inter alia that the grease service life and thus the bearing service life is reduced.
Highly efficient cooling systems have to date been achieved for instance in open-circuit ventilated motors. On account of the open housing, only the protective system IP 23 can be retained here. With closed housings, an efficient cooling system can be achieved for instance by means of a so-called ‘thermisiphon’ in the rotor. This guides heat effectively outwards, without it being necessary to open the housing.
Furthermore, the publication U.S. Pat. No. 4,574,210 A describes a motor with an external rotor and a corresponding cooling system. The internal stator has a hollow shaft, through which coolant can flow. Furthermore, the coolant flows around the external rotor. A tube with a flange-type plate is placed in the hollow shaft of the stator. Cooling air then flows through the tube into the hollow shaft and back outwards between the tube and the inner wall of the hollow shaft. There the coolant is carried along by the main cooling flow, which flows past the plate to the external rotor.
A similar electric machine is described in the publication U.S. Pat. No. 3,445,696 A. There the coolant flows into the hollow shaft of a rotor, reverses inside the hollow shaft and is drawn by an injection nozzle radially outwards into the cooling flow which cools the external stator.